In a conventional internal combustion piston type engine, when the explosion occurs in a cylinder, the piston is at or near its top dead-center position and, therefore, the torque exerted on the crank shaft is at a minimum when the explosion force is at a maximum. Therefore, the efficiency of such engine is rather poor. Also, in conventional two-cycle engines, the sweeping-away of the burned gases, after the detent, is inefficient, and this results also in poor efficiency.